Electric-Field-Controlled Interconversion of Antiferromagnetic States in a Two-Dimensional Antiferroelectric Halide Perovskite.

Electric-field control of spin-dependent electronic structures in antiferromagnets is highly desirable for low-power spintronic devices, however, such control remains largely unexplored. Here, we propose a promising route based on electric-field-driven switching between symmetry-distinct altermagnetic (AM) and type-IV antiferromagnetic (AFM) states, enabling reversible interconversion between spin-split and spin-degenerate electronic structures. First-principles calculations identify the organic-inorganic halide perovskite multiferroic monolayer ( C 2 H 5 NH 3 ) 2 [ FeCl 4 ] as a candidate platform for realizing this concept. This monolayer possesses two inequivalent antiferroelectric (AFE) structures that host distinct AFM electronic states: one realizes an AM state with nonrelativistic spin splitting, whereas the other belongs to the type-IV AFM class with spin-degenerate bands in the nonrelativistic limit. Remarkably, an in-plane electric field as low as 0.017 V/Å drives a transition between the two AFE phases, thereby switching the system between an AM spin-split state and a spin-degenerate type-IV AFM state. Moreover, the two AFM states exhibit distinct, symmetry-governed Kerr rotation responses, enabling optical detection of the electric-field-driven interconversion. These findings establish AFE interconversion as a ferroic-order-based route for electrically programming symmetry-distinct AFM electronic states, providing a new route for low-power, optically readable AFM spintronics.